tvheadend: bump to version 3.4

Update tvheadend to version 3.4

Beside many improvements, the new release has support for Timeshift. But unfortunately it
seems to have a memory leak when using Live TV (see https://tvheadend.org/issues/1713)  

A new patch was necessary to implement atomic functions for systems where native GNU atomic functions are not available (e.g. ARM based diskstations).

The package can be compiled for arch: 88f6281, ppc854x and x86
Tested on arch 88f6281 (DS213)